Light emitting diodes (LEDs) are semiconductor light emitting elements that convert electrical energy into ultraviolet light and visible light thereby to emit the converted light. Such light emitting diodes have a long service life and high reliability, and thus require less frequent replacement when being used as a light source, which is advantageous. LED lamps including an LED chip sealed with a transparent resin or the like are widely applied to: backlights of liquid crystal displays used in, for example, display portions of portable communication equipment, PC peripheral equipment, office automation (OA) equipment, and home electrical equipment; and lighting apparatuses such as signaling equipment, various switches, in-vehicle lamps, and general lighting.
With regard to the color tone of light emitted from the LED lamps, the LED chip is combined with various phosphors having different light emission colors, whereby light ranging from blue color to red color in a visible light region can be achieved in accordance with one's intended use. In particular, LED lamps that emit white light (white LED lamps) are rapidly becoming widespread to be applied to backlights of liquid crystal displays, in-vehicle lamps, and the like, and are expected to significantly expand as an alternative to fluorescent lamps in the future. For example, general fluorescent lamps are made using mercury, and hence the white LED lamps made without using mercury are considered to replace the fluorescent lamps in the future.
Known examples of the white LED lamp that has become widespread or is experimentally used at present may include: an LED lamp including a blue LED in combination with a yellow phosphor (such as YAG); and an LED lamp including an ultraviolet LED or violet LED having an emission wavelength of 360 to 440 nm in combination with a mixture of blue (B), green (G), and red (R) phosphors (BGR phosphors). Owing to superior luminance characteristics of the former, the former is currently more widespread than the latter. The light distribution of the former white LED lamp is, however, biased toward blue components and yellow components, and light containing red components is lacking and insufficient. Hence, in the case of even an LED lamp having emission chromaticity sufficient as a light source, the color of reflected light when an object is observed using this light source is significantly different from a natural color observed under sunlight. That is, there is posed a problem such that the color rendering properties of the former are low.
In contrast to the former, the latter white LED lamp including the ultraviolet LED, which unfortunately has luminance inferior to that of the former, has smaller color unevenness in emitted light and projected light, and thus is expected to become a mainstream white lamp in the future. As to the white LED lamp including the ultraviolet LED, lamp characteristics such as luminance (brightness) and color rendering properties are increasingly improved by adjusting characteristics of phosphors and a combination of the phosphors (see Patent Documents 1 and 2). For example, in order to enhance the brightness of the white LED lamp, it is studied to use a yellow phosphor having an emission peak wavelength of 540 to 570 nm in place of a green phosphor having an emission peak wavelength of 500 to 530 nm.
The white LED lamp to which such a mixture phosphor (BYR phosphor) containing the yellow phosphor in place of the green phosphor is applied has higher brightness than that of the white LED lamp containing the BGR phosphor, and thus is expected as a light source for a lighting apparatus. With regard to such a conventional white LED lamp to which the BYR phosphor containing the yellow phosphor is applied, however, a sufficient improvement effect of characteristics thereof cannot necessarily be obtained, and hence further enhancement in luminance and color rendering properties is required for the white LED lamp. Meanwhile, various yellow phosphors that can be used in combination with a blue LED have been proposed.
Known examples of the yellow phosphor that can be used in combination with the blue LED include: a cerium-activated yttrium aluminate phosphor (YAG), a cerium-activated terbium aluminate phosphor (TAG), and a europium-activated alkaline-earth silicate phosphor (BOSS) (see Patent Document 3). With regard to the conventional yellow phosphors, light emission characteristics of the yellow phosphors when being excited by blue light (emission wavelength: 430 to 500 nm) emitted from a blue LED have been studied. On the other hand, light emission characteristics of the yellow phosphors when being excited by light (emission wavelength: of 360 to 440 nm) emitted from an ultraviolet LED or violet LED have not been sufficiently studied, and thus are required to be studied and improved.
In addition, in order to enhance the color rendering properties of white light, a white LED apparatus including a blue LED and an ultraviolet LED in combination with a green phosphor and a red phosphor has also been proposed (see Patent Document 4). The color rendering properties of this white LED apparatus have been improved to some extent. However, because a part of blue light is easily absorbed by the phosphors, there is caused a disadvantage such that the luminance from a light emitting portion easily decreases.